#define _CRT_SECURE_NO_WARNINGS 1
#include <cmath>
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include<math.h>
#include<stdlib.h>
#include<stdio.h>
#include<unordered_map>
using namespace std;


  struct ListNode {
     int val;
      ListNode *next;
      ListNode() : val(0), next(nullptr) {}
      ListNode(int x) : val(x), next(nullptr) {}
      ListNode(int x, ListNode *next) : val(x), next(next) {}
  };
  class Solution {
  public:
      ListNode* swapPairs(ListNode* head) {
          if (head == nullptr || head->next == nullptr) return head;
          ListNode* hd = new ListNode();
          hd->next = head;
          ListNode* prev = hd;
          ListNode* cur = hd->next;
          ListNode* next = cur->next;
          ListNode* nnext = next->next;
          while (cur && next)
          {
              prev->next = next;
              next->next = cur;
              cur->next = nnext;
              prev = cur;
              cur = prev->next;
              if (cur)  next = cur->next;
              if (next) nnext = next->next;
          }

          ListNode* tmp = hd->next;
          delete hd;
          return tmp;
      }
  };

int main() {
    return 0;
}